High-frequency electric transmission system



Jime 4, 1946 D. c.- ESPLEY- 2,401,344

HIGH FREQUENCY ELECTRIC TRANSMISSION SYSTEM Filed June '7, 1941 FIGJ FlG.2

P 6 5 A A l SI t T I 1 1 I l I l I I INVENTOR DENNIS C LARK ETPLEY HIGH-FREQUENCY ELECTRIC TRANSMISSION SYSTEM Dennis Clark Espley, North Wembley, England,

assignor to The General Electric Company Limited, London, England Application June 7, 1941, Serial No; 397,016 In Great Britain June 14, 1940 exceed metres and wherein one part of the path along which the signals pass is rotatable relatively to another part of the path. In apparatus to which the invention relates more especially the said wave-length is less than 1 metre.

In order to make the invention more readily comprehensible, the application of it to one particular problem will be considered; but it will be evident that many features of this problem are not essential to the invention and, on the other hand, that the invention is applicable to problems having only a few features in common with that discussed.

In this problem the apparatus is a radio lighthouse. A stationary oscillator supplies a pair of directional radiators rotatable about a vertical axis and adapted to emit radiation in opposite directions perpendicular to that axis. Oscillations have to be communicated to the radiators from the oscillator through. the junction between the stationary and the rotating part. The lighthouse has to emit on one side of a vertical plane only, so that, if both radiators were operating continually, one would always be wasting energy in a useless direction. It is therefore required to switch each radiator ofi as it turns towards the useless side of the plane and to switch it on as it turns towards the useful side.

A conductive connection between the rotating and stationary parts, involving any form of rubbing contact, is unlikely to have continuously a small impedance to high frequency oscillations. An inductive connection through an ordinary condenser is an obvious alternative; but there are certain objections to it which will be clear to those skilled in the art. One object of this invention is to provide an alternative and usually preferable connection between the stationary and rotating parts.

The solution proposed depends on a well known property of concentric lines without dissipative loss whose length is m4 where )i is the wavelength of the oscillations along the line and b is an odd integer. Z0 is the finite characteristic impedance of the line, Z2 the impedance of a twoterminal network connected acrossone end of the line, Z1 the impedance looking into the other end of the line. The property is th'at Z1Zz=Zo In particular if Zz=, so that the first said end of the line is open, Zl=0, and the line, looked into from the other end, will appear short-cir- 2 cuited.' In this case there is no distinction between the two ends of the line;- it will appear short-circuited when looked into from either end.

For future reference it may be observed here that, if Z2 is zero and'one end is short-circuited, Z1

will be infinite, so that the presence of the line has no eiiect on any circuit across which it is connected at the other end.

In stating the foregoing property, it has been implied, as is usual in theoretical treatises, that it is immaterial what is the value of 11, so long as it is an odd integer. This is true so long as the oscillations have strictly a single frequency. In practice, oscillations have a finite band-width; the terminal impedance is then never quite zero or infinity, for the length cannot be pX/l for all the frequencies in the band. When they have a finite band-width, the optimum value of an is usually -1, if A is the wave-length corresponding to the centre of the band. I know of no reason why 1) should have any other value than 1, and therefore q (mentioned below) any value other than 1.

nected to it; 3, ii are the outer and inner members of another concentric line, coaxial with i, 2, rotatable relatively to it about the common axis and connected to the radiators. lines approach, member I is widened at I to surround a part 3' of member 3, and member 4 is made tubular at 4 so as to surround a reduced part 2' of member 2. The length of the overlapping portions is 7\/4. Then the parts I, 3' form a concentric line open at both ends; so do 2', 0'. Consequently these parts present zero impedance to the oscillations travelling in either direction along the main lines I, 2, 3, 4; I will appear directly connected to 3 and 2 will appear directly connected tot.

It is not really necessary that the input im,-

pedance X1 to the line I', 3' and the input impedance Xato the line 2', 4 should each be zero: all that is necessary is that the sum .X1+Xz should be zero. It is known that,-if X is the input impedance of an open-ended concentric line of constant characteristic impedance and of length l, X varies with l and, according t the Where these.

3 value of 1, may be a positive or a negative imaginary. Accordingly it the lines I, 3' and 2', 4' have the same, characteristic impedance, we can choose their lengths l1, 12 so that X1+m=0.

The necessary and sufllcient condition is that l1+l:=qA/2, where q is an integer which may be odd or even. If the two lines have diiierent characteristic impedances, as is probable, there will still be some relation between these lengths which ensures again that'X1+&- *0.

According to the invention, in apparatus of the typ specified the adjacentparts of the paths rotatable relatively to each other are each a concentric line, these lines being coaxial, the outerconductors of the concentric lines are arranged so that they together make up a concentric line A and the inner conductors of the concentric lines are arranged so that they form another concentric line B, and the lengths of the lines A and B are so related to the characteristic impedances of the lines A and B that X1+X:=0, where X1 is the input impedance to the line A and X2 is the input impedance to the line B. The said input impedances are, of course, those at the ends of the line at which the oscillations enter the lines.

contacts.

One way of providing such a switch is illustrated diagrammatically in Figure 2 of the accompanying drawing. The principle involved is not entirely new; indeed if the description about to be given were modified by making the plate 9 touch, and establish a conductive connection between, the parts 1 and 8 (or 1' or 8), the device would be one already known and used for other purposes.

Here a concentric line (which may be the line denoted by 3, 4 in Figure 1), made up of outer conductor 3 and inner conductor 4, and having the characteristic impedance Z0, leads the oscillations from the generator through the junction between the fixed and rotating parts. At the end remote from the junction it branches at a T into two concentric lines, each similar to the single line, having outer conductors 5 and 5' andinner conductors 6 and 6'. At a distance x/4 from the main T and on each side of it, a side concentric line or stub goes oil, having outer conductor 1 or I and inner conductor' 8 or 9'. The length of each of these lines is V4. Each of the lengths said to be M4 might be a multiple of V4 if certain consequential alterations were made. But, in view of what has already been said concerning such alterations, which are obvious to those skilled in the art, this possibility need notb'e discussed.

The structure just described is rotating bout the axisl. A fixed plate 9 is provided which, as the structure rotates, moves across and very close to the open ends of the cylinders I and 1'.

is therefore effectively hort-circuited at a distance A/ i from its junction from the single part of the T. The entrance to the arm 5', 6 is therefore of high impedance; little of the energy arriving along the single part of the line tends to go down the branch 5', 6' to the radiator at' the end of it; nearly all of it goes down the branch 5, 6. When the structure rotates till the plate 9 is opposite the end of l, the position is reversed; most of the energy goes down the 4 arm 5', 6'. The-plate 9 therefore acts as a switch transferring the energy alternately to the two radiators.

Accordingto a subsidiary feature of the invention, in apparatus of the type specified means for directing the said oscillations cyclically along a plurality of alternative paths as the two said parts of the path rotate relatively to each other compriseone or more concentric lines each of whose lengths is substantially rA/4 (where r is an integer which may be different for different lines) cooperating during the rotation with a, conductor relative to which they rotate and which theydo not touch, 50 that the input impedance of the line at one end becomes alternately large and small.

If the conductor does not touch the end of the line it will never make the shunt impedance at that end really zero or the input impedance at the other end substantially infinite. A device will now be described by which the input impedance at the unshunted end may be made substantially infinite.

The shunt impedance Z2 provided by the conductor will be capacitative, that is to say of .the form l/iwC, where w=21rx frequency. Since Z0- is real, positive and independent of u, Z1 is of the form a'wL. Suppose now that there is connected across the line in parallel with Z1, a capacity C. Then the total impedance Z1 across the line at that end is the said object is attained. But if, as is desirable, C is permanently in place and does not vary with Z2, it must be considered what is its effect, when Z2 is made infinite by the removal of the conductor. Then Z1=0; Z1 is the impedance of two parallel impedances of which one is zero; it is therefore also zero. Accordingly the presence of 0' makes no difference to the statements already made about what happens when the conductor is'removed. The capacity C' is represented by the dotted lines, as shown in Fig. 2, and may be provided for example by an adjustable plunger P, as shown in said figure.

Finally it may be pointed out that whereas the invention has been described with reference to a transmitter, it is likely to be of even greater utility in receivers, the said oscillator being replaced by a detector and the said radiators by receiving aerials. For in receivers it is likely to be more important to avoid rubbing contacts.

It must be understood that my invention is not limited to the particular examples hereinbe- V fore described and has the wider scope defined by the following claims:

1. In a, high frequency electric transmission system a feeder for transmitting high frequency electric oscillations between parts rotatable relative to each other, said feeder consisting of a pair of coaxial concentric lines rotatable relatively to each other, the outer conductors of these concentric lines being arranged so that they together made up a concentric line A and the inner conductors of the concentric lines being arranged so that they form another concentric line B, and the lengths of the lines A and B being so related to the'characteristic impedances of the lines A and B that X1+Xa=0, where X1 is the input impedance to the line A and X: is the input impedance to the line B. I

2. In a high frequency electric transmission system according to claim 1 means for directing the electric oscillations conveyed by the said feeder cyclically along a plurality of alternative paths as the two parts of the feeder rotate. relatively to each other, wherein the said means comprises at least one concentric line whose length is substantially 11/4, where r is an integer and A is the wave-length of the said osnately very large and small. DENNIS cillations, said concentric line being branched ofi from the said feeder and cooperating during the rotation with a conductor relative to which it rotates but which it does not touch, so that the input impedance of the line at one end becomes alternately large and small.

3. In a high frequency electric transmission system according to claim 1, means for directing the electric oscillations conveyed by the said feeder cyclically along a plurality of alternative paths as the two parts of the feeder rotate relatively to each other, wherein the said means comprises at least one concentric line whose length is substantially rx/4, where r is an integer and A is the wave-length of the said oscillations, said concentric line being branched off from the said feeder and cooperating at its free end during the rotation with a conductor relative to which it rotates but which it does not touch and wherein there is permanently shunted across the other end of that line a capacity compensating for the capacity existing between the said conductor and line when cooperating, so that the input impedance of the line at said other end becomes altercLAaK ESPLEY. 

